In recent years, lithium ion secondary batteries using, as their negative electrode active material, a lithium titanium complex oxide such as Li4Ti5O12 have been developed. For such lithium ion secondary batteries, it is known that carbon dioxide or lithium carbonate (Li2CO3) adsorbed onto their negative electrode active material is incorporated, as an impurity, into the negative electrode active material. When such a lithium ion secondary battery is stored at room temperature or a higher temperature, hydrogen fluoride (HF) generated by the hydrolysis of its electrolyte reacts with the impurity in its negative electrode to generate a carbon oxide type gas such as carbon monoxide and carbon dioxide. The generation of the gas causes a problem of the increased self-discharge, and degradation of large current characteristic due to the increased internal resistance.
It is known about lithium ion secondary batteries each having a carbon type negative electrode that a solid electrolyte interface (SEI) coat (hereinafter referred to as a “coat”) is present on the surface of the negative electrode active material. This coat is generated mainly by a reductive decomposition of the nonaqueous electrolyte so as to have a function of restraining reaction between the active material and the nonaqueous electrolyte. However, in the negative electrode of which a lithium insertion-extraction reaction potential is higher than the oxidation-reduction potential of lithium by a value more than 1 V, a coat as described above, for protecting the surface of the active material, is not easily formed so that side reactions, such as gas generation, are not easily restrained.